This project will (1) explore the circumstances under which processing of a visual input will be disrupted by a second, rapidly following input which neither overlaps nor is adjacent to the position of the first, and (2) will consider the hypothesis that developmental differences in this phenomenon contribute to children's limited short term memory. The disruption of initial visual processing of an input by a second non-overlapping visual stimulus is a form of visual masking which differs from more conventional types in that it does not require close spatial proximity between the mask and the initial input. It is labelled cogntive masking because cognitive demands of processing the second stimulus are thought to determine the masking effects. Factors whose relevance to cognitive masking will be assessed include the degree to which the individual is required to process the second input, and the degree of physical similarity between the two inputs. Two models of cognitive masking will be compared. One proposes that a limited attentional capacity is divided between maintaining relatively uncoded information immediately after receptor stimulation and taking in information from new visual inputs. A second model proposes that inputs from the second stimulus to the pattern analyzing mechanisms processing the first input cause disruption of the analysis. Task differences and developmental differences in cognitive masking will be explored to evaluate the hypothesis that cognitive masking is more likely to occur when the second input involves difficult visual discriminattons and is less likely to occur with the development of more efficient visual processing strategies.